The effect of annealing on the characteristics of semiconducting BaTiO3positive temperature coefficient of resistance devices

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Abstract
BaTiO3 positive temperature coefficient of resistance (PTCR) ceramics were prepared using commercially available BaTiO3 which was doped with 0.4 mol.% Ho to make it semiconducting. The samples were sintered in air at 1320 degrees C and while cooling to room temperature, different samples were annealed at 1220 degrees C for various periods ranging from 0 to 27 h. The annealing process was found to result in the formation of a resistive grain boundary region in series with the space charge layer. Dielectric measurements in the audio- and radio-frequency regions revealed that the resistive layer did not diffuse throughout the entire grain bulk, even during the longest annealing period. It was also found that annealing at 1220 degrees C results in (i) an increase in the value of rho max, (ii) a reduction in the temperature corresponding to rho max and (iii) an increase in the steepness of the resistivity-temperature plots in the region of the transition from low to high resistance. These changes were explained in terms of an increase in the acceptor state density.

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